Electron transfer reactions between proteins control the generation, flow and use of energy in biological systems. Central to many of the processes involved in these events are the cytochromes c. A major goal of this proposal is to contribute to the understanding of the solution structure and dynamics of cytochrome c both free and in complex with biologically relevant structures. The studies proposed here will augment, unify and clarify a wide body of information regarding the function of this essential protein. Experiments are proposed to directly and comprehensively determine the redox dependent structural and dynamical changes in cytochrome c, to quantify structural differences between two divergent eucaryotic cytochromes c, to test proposed molecular models for the structure of the complex between cytochrome c and cytochrome b5 and to examine the structural changes that occur upon association of cytochrome c with lipid. AH of these studies will bear directly upon the biological function of this protein and will contribute to the understanding of the properties governing electron transfer between proteins. In particular, these studies will address the apparent conflict between theoretical and experimental estimates of fundamental parameters governing electron transfer events. These studies will make extensive use of multinuclear and multidimensional NMR spectroscopy, a variety of distance geometry techniques for the determination of the solution structures of proteins and will employ detailed analysis of NMR relaxation to quantitate internal motion in these systems.